The present invention relates generally to a dot line printer. More particularly, the invention relates to an improvement of a dot line printer disclosed in U.S. Pat. No. 4,889,052 assigned to the same assignee.
A conventional dot line printer has an arrangement as shown in FIGS. 1 and 2.
A hammer bank 8 is movably disposed along a line extending in a direction transverse to a sheet of paper 9. The hammer bank 8 accommodates a plurality of print hammers 16. Each print hammer 16 is in the form of an elongated leaf spring having an upper end to which a dot pin is attached and a lower end secured to a mounting plate by means of, for example, screws. The print hammers 16 are arranged in spaced apart relation to one another in the direction in which the hammer bank 8 moves. Although not shown, a print hammer driver is provided in association with each print hammer 16, which includes a permanent magnet, a yoke and a release coil. The hammer 16 is attracted to the face of the yoke pole by the permanent magnet and is released therefrom in response to the energization of the release coil, whereby the dot pin strikes the paper through an ink ribbon 20 to thus make an impression of a dot on the paper 9.
The hammer bank 8 is moved by a shuttle mechanism including a shuttle motor 1. Specifically, an eccentric cam 2 is attached to the shaft of the shuttle motor 1. Two cam followers 5 having the same outer contours are rotatably supported at the tip ends of the arms of a U-shaped shift plate 4 and are in contact with the cam surface. The shift plate 4 is fixedly secured to a shift shaft 3 which in turn is slidably movably supported by a frame 7 through bearings 6. The shift shaft 3 is secured to one side face of the hammer bank 8 and a bank shaft 17 is secured to another side face thereof. The bank shaft 17 is slidably movably supported by a holder 18 through a bearing 19.
The print paper 9 is provided with uniformly-spaced perforations which are drivingly engaged by pin feed tractors 15 to move the paper 9 incrementally past the hammer bank 8 in a direction perpendicular to the direction in which the hammer bank 8 moves. The tractors 15 are driven by a paper feed motor 10. Rotations of the motor 10 are transmitted to the tractors 15 via pulleys 11, 13 on which a belt 12 is mounted, and a shaft 14.
When the shuttle motor 1 is energized, the hammer bank 8 shuttles back and forth along a print line. During the rightward movement of the hammer bank 8, the print hammers 16 are selectively fired, thereby making dot impressions on the paper 9. The hammer bank 8 reaches the rightmost position and turns around thereat. At this time, the paper feed motor 10 is energized to advance the paper 9. The hammer bank 8 then moves leftwardly and the print hammers 16 makes another dot impressions on the paper 9. In this manner, one line made up of plural dot lines is printed by repeatedly carrying out such alternate print and paper feed cycles.
U.S. Pat. No. 4,889,052 proposes a dot line printer with an increased printing speed wherein 6 dot lines are printed at a time during one print cycle and a line space feeding is accomplished during the reversing time of the hammer bank 8 at the leftmost or rightmost position. As such, the proposed dot line printer is capable of printing more than 300 lines of Kanji character strings per one minute.
However, the proposed dot line printer is still unsatisfactory. In the case where one print line is composed of 30 dot lines in which 24 dot lines are allocated to print positions, the hammer bank 8 needs to shuttle twice or to perform four scans to print one print line. Assuming that the hammer bank 8 starts moving from the leftmost position, six dot lines (1st to 6th dot lines) are simultaneously printed with the first scan from left to right. When the hammer bank 8 turns around at the rightmost position, the paper 9 is advanced 6 dot lines. During the second scan of the hammer bank 8 moving from right to left, the subsequent 6 dot lines from 7th to 12th dot lines are printed. When the hammer bank 8 turns around at the leftmost position, the paper 9 is advanced another 6 dot lines. Similarly, during the third scan of the hammer bank 8 moving from left to right, the 13th to 18th dot lines are simultaneously printed and the paper 9 is advanced further 6 dot lines. In the fourth scan of the hammer bank 8 moving from right to left, the 19th to 24th dot lines are printed. After the fourth scan, the paper 9 is advanced 12 dot lines in the reversing duration of the hammer bank 8 at the leftmost position. Thus, a line spacing between the present and the subsequent print lines is preserved and the subsequent print line is allowed to be printed thereafter.
In the above-described printing sequence, the paper feed amount in the first to third reversing durations is lesser than that in the fourth reversing duration, although the hammer bank reversing duration is the same. If a paper feed time is set to meet a lesser amount of paper feed, subsequent line printing starts before the feeding of the paper 9 is completed, with the result that the printing quality is degraded. On the other hand, if the paper feed time is set to meet a larger amount of paper feed, the printing quality is not degraded. However, subsequent line printing does not immediately take place despite the fact that the line space feed has already terminated. This delay causes degradation to the printing speed.
As an improvement of the above-described dot line printer, it has been proposed that the hammer bank reversing duration be shortened only when the hammer bank turns around at the leftmost position. This proposal is made in view of the fact that the line space feeding is always performed when the hammer bank 8 turns around at the leftmost position. Accordingly, the printing speed can be increased somewhat.
However, the second proposal is still not ideal in that 12 dot lines of feeding time is given to the secondly performed hammer bank reversing for effecting 6 dot lines of feeding. Moreover, when printing is carried out while skipping some of the lines, it takes a long time to feed the paper for the skipped lines. The paper feeding will not be completed before the hammer bank 8 performs the subsequent scan. If the subsequent line printing were initiated from the rightward scan of the hammer bank 8, the subsequent line space feeding would be performed in the rightmost position in which the reversing duration of the hammer bank 8 is set shorter. In actuality, however, the line space feeding cannot be performed at the rightmost position, and so the printing cannot be started until the hammer bank 8 turns around at the leftmost position. Consequently, this unnecessary movement of the hammer bank results in lowering the printing speed. As described, when skipping of more than 2 lines after printing one line, unnecessary movement of the hammer bank is to be performed, and as a result, the printing speed would be excessively lowered.